The objective of this proposal is to better define the role of the activated macrophage in host resistance to cancer and intracellular infection. The goal is to continue and extend our studies of basic mechanisms of nonspecific resistance in experimental animals. Our completed studies in mice show that mice prophylactically infected with intracellular protozoan parasites have remarkable nonspecific resistance to autochthonous and transplanted tumors. We now plan to use the same experimental model to determine if nonspecific resistance to neoplasia can be stimulated by infection with intracellular protozoa after the graft of tumor cells. In addition, to better define nonspecific factors of host resistance and relate them to specific immune mechanisms, a profile of a number of immunologic and nonimmunologic parameters will be measured. All studies are designed to lead to a better definition of nonspecific mechanisms in the homeostatic control of carcinogenesis and tumor growth.